A cellular or trunked communication system is one in which mobile or portable user terminals, such as mobile telephones, portable radios or radios on vehicles, herein collectively referred to as ‘mobile stations’, can communicate via a network infrastructure which generally includes fixed installations including a plurality of fixed base stations (base transceiver stations) and various sub-systems for management and control of the system including the base stations. Each base station has one or more transceivers which serve mobile stations in a given region or area known as a ‘cell’ or ‘site’ by radio communication. The cells of neighbouring base stations are often overlapping.
Generally, it is desirable for the mobile stations to be served by a base station which can provide good radio signals to and from the mobile station. Since mobile stations can move from one region to another it is known for the mobile stations to monitor signals from different base stations, to operate a procedure to determine which base station can best serve the mobile station, to determine whether it would be worthwhile to switch from a current serving base station to another one and if appropriate to carry out such a switch. In the art, the procedure to monitor for and to make determinations regarding a possible switch is known as a ‘cell re-selection’ procedure. This usually operates in two stages, namely (i) a first stage in which the mobile station draws up a list of candidate non-serving base stations to which it could potentially switch to as its serving base station, including a preferred non-serving base station which is selected to be top of the list; and (ii) a second stage in which measurements are carried out comparing properties of the preferred non-serving base station with those of the current serving base station and determining whether certain criteria are met which require a switch to the preferred base station to be made. Where such criteria are met, a procedure known as ‘handover’ or ‘handoff’ is carried out to effect such a switch.
There are various parameters which are used in a cell re-selection procedure in known systems to allow a mobile station to select between base stations either in the first stage or in the second stage referred to above. The parameters used may for example be selected from: (i) received signal strength from a given base station at the mobile station; (ii) signal quality (e.g. bit error rate) of a signal received at the mobile station from a given base station; (iii) subscriber class (explained below); (iv) cell priority (explained below); (v) services available from a given base station/cell; (vi) preferred cell of the mobile station (e.g. its ‘home’ cell); and (vii) cell loading of a given cell supported by a particular base station.
For example, in TETRA systems, i.e. those that operate in accordance with TETRA (Terrestrial Trunked Radio) standards defined by the European Telecommunications Standards Institute (ETSI), the parameters used by a mobile station include recorded received signal strength (energy) values plus some service parameters. Each base station continuously broadcasts to mobile stations served by that station information relating to neighbour base stations which could potentially serve the mobile stations. The information is broadcast on a control channel. The information includes channel frequencies and other data which allows each mobile station easily to search for other base stations. Each mobile station runs an algorithm which produces a list of suitable neighbouring base stations from which it can select a base station to which it could undergo handover.
Allocating each mobile station into a ‘subscriber class’ as referred to above is a known grouping technique. The population of mobile stations may be split say into three classes A, B and C. Some sites may advertise that they only allow or prefer class A. A mobile station that is configured as a member of Class C will not use or prefer that site. The TETRA standard supports use of 16 such subscriber classes.
The ‘cell priority’ referred to above indicates whether a cell is a ‘priority cell’ or not. Some cells may be designated as ‘priority cells’ within the system management. If a mobile station in the process of selecting a cell can detect a priority cell and a non-priority cell then the priority cell should be selected in preference. An application of priority cell could be in an ‘umbrella cell’ scenario: for example, say there are several ‘regular’ cells and one ‘umbrella’ cell. If a mobile station can detect a regular cell then this should be selected as a priority cell. The umbrella cell is a non-priority cell and should be selected only if no regular cells can be detected, so that the umbrella cell does not become overloaded.
When a mobile station determines in the second stage of cell re-selection that the link with the current serving base station as defined by one or more of the parameters described above should be exchanged with a better link it abandons the link with the current serving base station and undergoes handover to the ‘best’ neighbour base station which is top of the list of non-serving base stations that it has compiled.
In some cellular or trunked systems, e.g. those used for communications in public safety services, it is usual for users to communicate in groups (often referred to as ‘talkgroups’). The population of mobile stations belonging to a particular group can be spread throughout the system. In other words, the mobile stations which are members of the group can be served by many different base stations. This spread of service throughout the system can result in less than optimal use of traffic channels in the system since a traffic channel has to be assigned at each base station where a mobile station is attached (served) to a particular group.